Expander device for isolating bus segments in I/O subsystem

ABSTRACT

An expander device and method for isolating bus segments from one another in an I/O subsystem. The expander device is arranged to couple the bus segments for communication in the I/O subsystem. The expander device includes a first I/O interface circuit, a second I/O interface circuit, and an expander controller. The first I/O interface circuit is configured to be coupled to a first bus segment and is adapted to interface input and output communication signals with the first bus segment. The second I/O interface circuit is configured to be coupled to a second bus segment and is adapted to interface the input and output communication signals with the second bus segment. The expander controller is coupled to communicate the input and output communication signals between the first and second I/O interface circuits. The expander controller is further arranged to control communication between the bus segments and includes a segment controller adapted to generate a first signal. The segment controller provides the first signal to the first and second I/O interface circuits to disable output of the communication signals from the first and second I/O interface circuits to the first and second bus segments.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.09/858,321 entitled “Expander Device and Method for Resetting BusSegments in I/O Subsystem Segmented with Expanders” by John S. Packer etal. This application, filed on the same day as the present application,are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to computer I/O subsystems havingperipheral bus segments, and more particularly to device and method forisolating bus segments in I/O subsystems.

2. Description of the Related Art

Modern computer systems often utilize one or more buses to connect toperipheral devices to enhance its resources. For example, the resourcesof a computer system may be substantially increased by connecting thecomputer system to one or more peripheral devices such as disk drives,tape drives, printers, scanners, optical drives, and the like. Theseperipheral devices are attached to the computer system by means of aperipheral bus (e.g., cable).

One of the most widely used peripheral buses is the well known smallcomputer systems interface (SCSI) bus, which is defined in conformitywith well known SCSI protocols (e.g., SCSI-1, SCSI-2, SCSI-3, etc.),which are incorporated herein by reference. The SCSI protocols aredesigned to provide an efficient peer-to-peer I/O interface between ahost computer and its peripheral devices in a computer system.

FIG. 1 shows a block diagram of a conventional computer system 100including a host computer 102, a plurality of SCSI devices 106, and anSCSI bus 108. The host computer 102 includes an SCSI host adapter 104for communicating with the SCSI devices 106. The host adapter 104 in thecomputer system 100 controls communucation between the host computer 102and the SCSI devices 106. For example, the host adapter 104 provides aphysical connection between the host computer 102 and the SCSI bus 108.In addition, it is configured to receive data, address, and controlsignals from the host computer 102 and convert the signals intocorresponding SCSI compatible data, address, and control signals.Conversely, the SCSI host adapter 104 is also configured to receive SCSIcompatible data, address, and control signals from the SCSI devices 106through the SCSI bus 108 and convert them into corresponding host-buscompatible data, 15 addressing, and control signals. The SCSI hostadapter 104 is well known in the art and may be implemented, forexample, by using AIC-7890A™ packaged semiconductor device, which isavailable from Adaptec Inc., of Milpitas, Calif.

Under the conventional SCSI specifications, the SCSI bus 108 may connectup to 16 SCSI devices including the host adapter 104 depending on thetype of SCSI bus implemented. The SCSI devices 106 may be peripheraldevices such as disk drives, tape drives, printers, scanners, opticaldrives, or any other devices that meet the SCSI specification. The SCSIbus 108 is typically implemented as a cable having a set of parallelwires. For example, the SCSI-1 cable has 50 wires. Of these 50 wires,eight wires are for data, one wire is for parity, nine wires are forcontrol, 25 wires are for ground, and the remaining wires are for poweror are reserved for future use. The eight data wires are used to carryeight bits of data in parallel. In general, conventional SCSI bus cablesinclude either 8 or 16 data wires for carrying 8 or 16 bits,respectively, of data in parallel. The data wires in the bus thus definea datapath for communicating bits equal to the number of data wires inthe bus.

A traditional SCSI bus may accommodate a plurality of SCSI devices up toa maximum number equal to the number of data bits in the SCSI bus. Inpractice, the width of the SCSI datapath is typically 8 or 16 bitscorresponding to the number of data wires in the SCSI bus. This meansthe maximum number of SCSI devices, including a host adapter, that canbe attached to an SCSI bus is limited to 8 or 16.

Conventional SCSI buses, however, are generally capable of transmittingsignals reliably for a specified cable length only. For example, SCSIbus cables conforming to SCSI specifications are typically limited to amaximum length such as 3, 6, 12, or 25 meters, and the like. In moderncomputer systems such as servers that need to communicate with numerousSCSI devices, some SCSI peripheral devices may be located more than thespecified cable length away from a host computer.

In such instances, an expander, which is essentially a repeater, istypically used to add another SCSI bus, which is then used to coupleadditional SCSI devices. FIG. 2 illustrates a conventional I/O subsystem200 that includes an expander 214 for connecting a pair of SCSI bussegments 208 and 210. In the I/O subsystem 200, a host computer 202 iscoupled to the SCSI bus segment 208 via an SCSI host adapter 204. One ormore SCSI devices 206 are coupled to the SCSI bus segment 208.Additionally, one or more SCSI devices 212 and a host computer 216 arecoupled to the SCSI bus segment 210 and the host computer 216 alsoincludes an SCSI adapter for communicating with the SCSI bus segment210. The expander 214 is coupled between the SCSI bus segments 208 and210 to regenerate signals received on either SCSI bus segments 208 or210 for transmission to the other SCSI bus segment. By so repeatingsignals, the expander 214 functions to extend the length of the overallSCSI bus so that additional SCSI devices 212, host computer 316, orother expanders can be attached to the SCSI bus segment 210.

Unfortunately, using the expander 214 as a repeater has severaldrawbacks. For example, when the host computer 202 is accessing a SCSIdevice 206 on its bus segment 208, the other host computer 216 typicallymay not access any SCSI devices 206 or 212. This is so even if the hostcomputer 216 needs to access only the SCSI devices 212 on its bussegment 210. To communicate with SCSI devices 212 on its own bus segment210, the host computer 216 must wait until the other host computer 216has completed accessing the SCSI device 206. As can be appreciated, thedelay in communicating with SCSI devices 212 on its own bus segment 210may cause substantial degradation in performance.

In view of the foregoing, what is needed is an expander device andmethod for for isolating bus segments in an I/O subsystem so that bussegments may operate independent of each other.

SUMMARY OF THE INVENTION

The present invention fills these needs by providing expander device andmethod for isolating bus segments in an I/O subsystem. It should beappreciated that the present invention can be implemented in numerousways, including as a process, an apparatus, a system, a device, amethod, or a computer readable medium. Several inventive embodiments ofthe present invention are described below.

In one embodiment, the present invention provides an expander device forisolating bus segments from one another in an I/O subsystem. Theexpander device is arranged to couple the bus segments for communicationin the I/O subsystem. The expander device includes a first I/O interfacecircuit, a second I/O interface circuit, and an expander controller. Thefirst I/O interface circuit is configured to be coupled to a first bussegment and is adapted to interface input and output communicationsignals with the first bus segment. The second I/O interface circuit isconfigured to be coupled to a second bus segment and is adapted tointerface the input and output communication signals with the second bussegment. The expander controller is coupled to communicate the input andoutput communication signals between the first and second I/O interfacecircuits. The expander controller is further arranged to controlcommunication between the bus segments and includes a segment controlleradapted to generate a first signal. The segment controller provides thefirst signal to the first and second I/O interface circuits to disableoutput of the communication signals from the first and second I/Ointerface circuits to the first and second bus segments.

In another embodiment, the present invention provides an SCSI expanderfor isolating SCSI bus segments in an SCSI I/O subsystem. The SCSIexpander is arranged to couple a first bus segment and a second bussegment in the SCSI I/O subsystem and is further configured to repeatcommunication signals by receiving the communication signals from oneSCSI bus segment and outputting the communication signals to the otherSCSI bus segment. The SCSI expander includes a first SCSI I/O interface,a second SCSI I/O interface, and an SCSI expander controller. The firstSCSI I/O interface circuit is adapted to interface communication signalswith the first SCSI bus segment while the second SCSI I/O interfacecircuit is arranged to interface the communication signals with thesecond SCSI bus segment. The SCSI expander controller is coupled tocommunicate the communication signals between the first and second SCSII/O interface circuits and is arranged to control communication betweenthe first and second SCSI bus segments. The SCSI expander controllerincludes a segment controller adapted to generate a first signal, whichis provided to the first and second SCSI I/O interface circuits todisable output of the communication signals from the first and secondSCSI I/O interface circuits to the first and second SCSI bus segments.The disabling of the output of the communication signals effectivelyisolates the first and second SCSI bus segments from one another in anisolation mode so that the communication signals received on one SCSIbus segment are not transmitted to the other SCSI bus segment.

In yet another embodiment, a method for isolating bus segments in an I/Osubsystem is disclosed. The I/O subsystem includes an expander coupledbetween a first bus segment and a second bus segment. The expander isconfigured to repeat communication signals by receiving thecommunication signals from one bus segment and outputting thecommunication signals to the other bus segment. In this method, theexpander receives an isolation command from a host computer on the firstbus segment. The isolation command received from the host computer isconfigured to instruct the expander to isolate the first bus segmentfrom the second bus segment. In response to the isolation command, theexpander operates in an isolation mode by preventing communicationsignals received on one bus segment from being output onto the other bussegment such that the first and second bus segments are isolated fromone another.

Advantageously, the expander and method of the present invention allow adual mode operation for the expander:isolation mode and normal repeatingmode. To enter a segment isolation mode, the expander detects anisolation command that instructs the expander to enter a segmentisolation mode from either bus segment. Preferably, the expander entersthe segment isolation mode in a controlled manner by entering the BUSFREE state before isolation is enabled. In the isolation mode, theexpander disables propagation of all signals from one segment to theother. In this mode, each bus segment on either side of the expanderoperates independent of the other.

To exit from the segment isolation mode, the expander detects an exitisolation command from either bus segment. The expander exits thesegment isolation mode in a controlled manner by waiting for the BUSFREE state on the far-side bus segment. The controlled entry and exit toand from the isolation mode protects the segment on the far-side, whichis the segment remote from the segment on which a command is received,against signal corruption. Other aspects and advantages of the inventionwill become apparent from the following detailed description, taken inconjunction with the accompanying drawings, illustrating by way ofexample the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed description in conjunction with the accompanying drawings, andlike reference numerals designate like structural elements.

FIG. 1 shows a block diagram of a conventional computer system includinga host computer, a plurality of SCSI devices, and an SCSI bus.

FIG. 2 illustrates a conventional computer system that includes anexpander for connecting a pair of SCSI buses.

FIG. 3 illustrates an exemplary I/O subsystem including an expander thatallows isolation of individual bus segments and in accordance with oneembodiment of the present invention.

FIG. 4 shows an exemplary method implemented in the I/O subsystem forentering into an isolation mode and exiting from the isolation mode inaccordance with one embodiment of the present invention.

FIG. 5A shows a more detailed flowchart of an operation for enteringinto an isolation mode in accordance with one embodiment of the presentinvention.

FIG. 5B illustrates a more detailed flowchart of the operation forexiting the isolation mode in accordance with one embodiment of thepresent invention.

FIG. 6 shows a more detailed schematic diagram of the expander inaccordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides expander device and method for isolatingbus segments in an I/O subsystem. It will be obvious, however, to oneskilled in the art, that the present invention may be practiced withoutsome or all of these specific details. In other instances, well knownprocess operations have not been described in detail in order not tounnecessarily obscure the present invention.

The present invention allows an expander to operate in dual modes toeither isolate bus segments in an I/O subsystem or function as arepeater by propagating communication signals received on one bussegment to the other bus segment. The I/O subsystem includes two or morebus segments and one or more expanders. Each expander couples a pair ofbus segments and is adapted to repeat communication signals received onone bus segment to the other bus segment. In this normal operating mode,the expander allows devices attached to one bus segment to communicatewith devices on the other bus segment.

The expander device and method of the present invention allow isolationof individual bus segments attached to an expander so that communicationsignals received from one bus segment is not transmitted to the otherbus segment attached to the expander. When an expander receives acommand to isolate a bus segment, the expander enters into an isolationmode to disable transmission of communication signals received from onebus segment to the other bus segment. The expander may also exit theisolation mode in response to a command to exit the isolation mode. Inthis event, the expander enables its signal propagation function so thatcommunication signals received from one bus segment can be transmittedto the other bus segment.

FIG. 3 illustrates an exemplary I/O subsystem 300 that allows isolationof bus segments 302 and 304 in accordance with one embodiment of thepresent invention. The I/O subsystem 300 includes one or more hostcomputers 306 and 308, one or more peripheral devices 310, 312, 314, and316, and an expander 318. Although the present invention is illustratedusing the I/O subsystem 300, it should be appreciated that the I/Osubsystem 300 may include any number of expanders, bus segments, hostcomputers, and peripheral devices. Preferably, the I/O subsystem 300 isan SCSI bus I/O subsystem with each device, host computer, and bussegment being adapted to recognize and operate under SCSI protocols.Those skilled in the art will, however, recognize that the I/O subsystem300 may also be implemented using any other suitable in-line busprotocols.

The I/O subsystem 300 defines two bus segments 302 and 304 that arecoupled to each other through the expander 318. Each of the bus segments302 and 304 is adapted to couple devices, preferably in an in-linefashion such as in SCSI bus. For example, the host computer 306 andperipheral devices 310 and 312 are coupled to the bus segment 302 whilethe host computer 308 and peripheral devices 314 and 316 are coupled tothe other bus segment 304. The expander 318 couples the two bus segments302 and 304 and is adapted to propagate communication signals receivedon one bus segment to the other bus segment during normal operation.

In this configuration, the bus segments 302 and 304 operate effectivelyas a single bus for the entire I/O subsystem 300 to allow each hostcomputer 306 or 308 to access devices located at the other side of theexpander 318. The I/O subsystem 300 of the present invention also allowsthe bus segments 302 and 304 to be isolated from each other so thatsignals are not transmitted from one bus segment to the other. Thisallows each of the bus segments 302 and 304 with attached devices andhost computers to operate independent of the other.

FIG. 4 shows an exemplary method 400 implemented in the I/O subsystem300 for entering into an isolation mode and exiting from the isolationmode in accordance with one embodiment of the present invention.Initially, the expander 318 coupling the bus segments 302 and 304 is innormal operating mode. In this mode, the expander 318 propagates signalsreceived from one bus segment to the other bus segment. For example, theexpander 318 repeats signals received on bus segment 302 to the otherbus segment 304 and vice versa.

The expander 318 then enters into an isolation mode in operation 404 inresponse to a command to isolate the bus segments 302 and 304 from ahost computer 306 or 308. In this isolation mode, expander 318 stopspropagating signals received from one bus segment to the other. Hence,the expander 318 effectively isolates the bus segments 302 and 304 fromone another for independent operation.

When bus segments 302 and 304 no longer need to be isolated, theexpander 318 exits the isolation mode in operation 406 in response to anexit isolation command from a host computer 306 or 308. Upon exiting theisolation mode, the expander 318 returns to the normal operating modeand resumes repeating signals received on one bus segment to the otherbus segment. The method 400 then terminates in operation 408.

FIG. 5A shows a more detailed flowchart of the operation 406 forentering into an isolation mode in accordance with one embodiment of thepresent invention. When bus segments 302 and 304 need to be isolated, ahost computer 306 or 308 selects the expander 318 that couples the bussegments 302 and 305 in operation 502. The host computer then instructsthe expander 318 to enter the isolation mode by issuing an isolationcommand in operation 504.

In response to the isolation command, the expander 318 enters theisolation mode and stops repeating signals in operation 506. Preferably,the expander 318 enters the segment isolation mode when far-side bussegment enters a BUS FREE state to prevent signal corruption. Forexample, if the host computer 306 has requested bus isolation, theexpander waits until it releases both segments 302 and 304 to the BUSFREE state. While operating in the isolation mode, the expander 318stops propagating signals received from one bus segment to the other sothat the bus segments 302 and 304 are effectively isolated from oneanother. On one side, for example, communication takes place only amongthe host computer 306 and peripheral devices 310 and 312 through the bussegment 302. On the other side, only the host computer 308 and theperipheral devices 314 and 316 communicate with each other via bussegment 304.

The expander 318 may exit from the isolation mode to operate in itsnormal operating mode. FIG. 5B illustrates a more detailed flowchart ofthe operation 406 for exiting the isolation mode in accordance with oneembodiment of the present invention. When bus segment isolation is nolonger needed, a host computer 306 or 308 selects the expander 318operating in the isolation mode in operation 552. Then, the hostcomputer instructs the expander 318 to exit the isolation mode bysending an exit isolation command to the expander 318 in operation 554.It should be appreciated that the expander 318 may exit from anisolation mode in response to an exit isolation command from either bussegment 302 or 304.

In response to the exit isolation command, the expander 318 exits fromthe isolation mode in operation 556 by enabling its signal repeatingfunction between the bus segments 302 and 304. Preferably, the timing ofthe expander 318 for exiting from the isolation mode is synchronized tothe BUS FREE signal on a bus segment that is located on the far side ofthe expander 318 relative to the requesting host computer. In oneembodiment, the expander 318 waits until a BUS FREE signal is assertedon the bus segment remote from the requesting host computer to startrepeating signals. For example, if the host computer 306 has sent theexit isolation command to the expander 318, the expander 318 waits untilthe bus segment 304 on the other side of the expander 318 enters into aBUS FREE state. Conversely, when the host computer transmitted the exitisolation command, the expander 318 waits until a BUS FREE signal isasserted on the bus segment 302 on the other side of the of the expander318.

Once the expander 318 has exited from the isolation mode, the expander318 is back in its normal operating mode. In the normal operating mode,the expander 318 propagates signals received on one bus segment to theother bus segment to allow communication between host computers andperipheral devices attached to the bus segments 302 and 304. The methodthen terminates in operation 412.

FIG. 6 shows a more detailed schematic diagram of the expander 318 inaccordance with one embodiment of the present invention. The expander318 includes an SCSI controller 504 and I/O interfaces 502 and 506.Although the expander 318 is illustrated using an SCSI controller 504,it should be appreciated that other types of expander controllers may beused in conformance with any suitable buses such as in-line buses.

The I/O interfaces 604 and 606 in the expander 318 provides interface tobus segments 302 and 304, respectively, for communicating data andcontrol signals. The I/O interface 604 is coupled between the bussegment 302 and the SCSI controller 602 to provide input and output ofthe data and control signals. Similarly, the I/O interface 604 iscoupled between the SCSI controller 602 to provide input and output ofthe data and control signals.

The I/O interfaces 604 and 606 include buffers for driving signals oneither side of the expander 318. Specifically, the I/0 interface 604includes an input buffer 612 and an output buffer 614. The input buffer612 receives communication signals from the bus segment 302 fortransmission to the SCSI controller 602 while the output buffer 614 isadapted to receive communication signals from the SCSI controller 602for output on the bus segment 302. Likewise, the I/O interface 604includes an input buffer 618 and an output buffer 616. The input buffer618 is adapted to receive communication signals from the bus segment 304for transmission to the SCSI controller 602 while the output buffer 616receives communication signals from the SCSI controller 602 for outputon the bus segment 304. In this arrangement, each of the buffers 612drives an input signal for output. Although the I/O interfaces 604 and606 are illustrated with an input buffer and an output buffer, it shouldbe appreciated that the I/O interfaces 604 and 606 may include anynumber of input and output buffer pairs, with each input and outputbuffer pair being arranged to transmit a signal.

The SCSI controller 602 is coupled receive and/or output data andcontrol signals to and from the buffers 612, 614, 616, and 618 in theI/O interfaces 604 and 606 and is adapted to provide control functionsfor the expander 318. The SCSI controller 604 includes a segmentcontroller 608 to control the operating mode (e.g., isolation mode, exitisolation mode, normal operating mode) of the expander 318.

During normal operating mode, the buffers 612, 614, 616, and 618 operateto drive input signals for output so that communication signals receivedon one bus segment is propagated onto the other bus. For example, when asignal is received from the bus segment 302, the input buffer 612 drivesthe signal onto the SCSI controller 602. The SCSI controller 602 thenprovides the signal to the output buffer 616, which drives the signalonto the bus segment 304. On the other hand, the input buffer 618 drivesa signal received on the bus segment 304 onto the SCSI controller 602,which provides the signal to the output buffer 614. The output buffer614 then drives the signal onto the bus segment 302. In this manner, theexpander 318 repeats signals received on one bus segment to the otherbus segment.

When the bus segments 302 and 304 needs to be isolated, a host computer306 or 308 issues an isolation command REQ_ISOLATE to the expander 318.The SCSI controller receives the isolation command, which is provided tothe segment controller 608. In response to the isolation command, thesegment controller 608 asserts an ISOLATE signal 610, which is fed tothe I/O interfaces 604 and 606.

The asserted ISOLATE signal 610 serves to disable output functions ofthe I/O interfaces 604 and 606 by disabling the output buffers 614 and616. Accordingly, the expander 318 does not output signals onto the bussegments 302 and 304. However, the input functions of the I/O interfaces604 and 606 are not disabled so that the expander 318 may receivecommunication signals (e.g., commands, data, control signals, etc.), forexample, to exit from the isolation mode. That is, the input buffers 612and 618 are not disabled so that the expander may receive communicationsignals from bus segments 302 and 304. By disabling of the outputfunctions of the I/O interfaces 604 and 606, the expander 318effectively prevents repeating or propagation of signals received on onebus segment to the other bus segment.

The expander 318 may exit from the isolation mode into its normaloperating mode in response to an exit isolation command,REQ_EXIT_ISOLATE from one of the host computers 306 or 308. When theexit isolation command REQ_EXIT_ISOLATE is received on either bussegment 302 or 304, the expander 602 provides the exit isolation commandto the segment controller 608. In response, the segment controller 608enables the output buffers 614 and 616 by deasserting the ISOLATE signalpreviously provided to the output buffers. Preferably, the segmentcontroller 608 deactivates the ISOLATE signal when the bus segmentremote from the requesting host computer is in BUS FREE state. Thisensures synchronous operation of the bus segments 302 and 304 on eitherside of the expander 318. With the output buffers 614 and 616 enabled,the expander 318 repeats signals between the bus segments 302 and 304 ina normal operating mode.

Thus, the expander device and method of the present invention allow adual mode operation for the expander in either isolation mode or normalrepeating mode. Although the foregoing invention has been described insome detail for purposes of clarity of understanding, it will beapparent that certain changes and modifications may be practiced withinthe scope of the appended claims. Accordingly, the present embodimentsare to be considered as illustrative and not restrictive, and theinvention is not to be limited to the details given herein, but may bemodified within the scope and equivalents of the appended claims.

1. An expander device for isolating bus segments from one another in anI/O subsystem, the expander device coupling the bus segments forcommunication in the I/O subsystem, the expander device including: afirst I/O interface circuit configured to be coupled to a first bussegment, the first I/O interface circuit being adapted to interfaceinput and output communication signals with the first bus segment; asecond I/O interface circuit configured to be coupled to a second bussegment and being adapted to interface the input and outputcommunication signals with the second bus segment; and an expandercontroller coupled to communicate the input and output communicationsignals between the first and second I/O interface circuits, theexpander controller being arranged to control communication between thebus segments, the expander controller including a segment controlleradapted to generate a first signal, wherein the segment controllerprovides the first signal to the first and second I/O interface circuitsto disable output of the communication signals from the first and secondI/O interface circuits to the first and second bus segments, thedisabling of the output of the communication signals isolates the firstand second bus segments from one another in an isolation mode so thatthe communication signals received on either bus segment are allowed butare not transmitted, the expander transitions from the isolation mode toa normal operating mode in order to repeat communication signals byreceiving the communication signals from one of the first bus segment orthe second bus segment and outputting the communication signals to oneof the second bus segment or the first segment.
 2. The expander deviceas recited in claim 1, wherein the expander device is adapted to receivethe communication signals from the first and second bus segments whilein the isolation mode.
 3. The expander device as recited in claim 2,wherein the segment controller generates the first signal in response toan isolation command received from the first bus segment.
 4. Theexpander device as recited in claim 3, wherein the segment controllerdeasserts the first signal to exit from the isolation mode.
 5. Theexpander device as recited in claim 4, wherein the segment controllerdeasserts the first signal when the second bus segment is in a bus freestate.
 6. The expander device as recited in claim 3, wherein the firstI/O interface circuit includes a first input buffer and a first outputbuffer and wherein the second I/O interface circuit includes a secondinput buffer and a second input buffer.
 7. The expander device asrecited in claim 6, wherein the first and second input buffers arearranged to drive the communication signals for input to the expanderdevice from the first and second bus segments, respectively, and whereinthe first and second output buffers are arranged to drive thecommunication signals for output from the expander device to the firstand second bus segments, respectively.
 8. The expander device as recitedin claim 7, wherein the first signal disables the first and secondoutput buffers to disable the output of communication signals to thefirst and second bus segments, respectively.
 9. The expander device asrecited in claim 1, wherein the first and second bus segments are SCSIbus segments and wherein the expander controller is an SCSI controller.10. An SCSI expander for isolating bus segments in an SCSI I/Osubsystem, the SCSI expander device coupling a first SCSI bus segmentand a second SCSI bus segment in the SCSI I/O subsystem, the SCSIexpander being configured to repeat communication signals by receivingthe communication signals from one SCSI bus segment and outputting thecommunication signals to the other SCSI bus segment, the SCSI expandercomprising: a first SCSI I/O interface circuit adapted to interfacecommunication signals with the first SCSI bus segment; a second SCSI I/Ointerface circuit adapted to interface the communication signals withthe second SCSI bus segment; and an SCSI expander controller coupled tocommunicate the communication signals between the first and second SCSII/O interface circuits, the SCSI expander controller being arranged tocontrol communication between the first and second SCSI bus segments,the SCSI expander controller including a segment controller adapted togenerate a first signal, wherein the segment controller provides thefirst signal to the first and second SCSI I/O interface circuits todisable output of the communication signals from the first and secondSCSI I/O interface circuits to the first and second SCSI bus segments,wherein the disabling of the output of the communication signalsisolates the first and second SCSI bus segments from one another in anisolation mode so that the communication signals received on either SCSIbus segment are allowed but not transmitted to the other SCSI bussegment, the expander transitions from the isolation mode to a normaloperating mode in order to repeat communication signals by receiving thecommunication signals from one of the first SCSI bus segment or thesecond SCSI bus segment and outputting the communication signals to oneof the second SCSI bus segment or the first SCSI segment.
 11. The SCSIexpander as recited in claim 10, wherein the SCSI expander is adapted toreceive the communication signals from the first and second SCSI bussegments while in the isolation mode.
 12. The SCSI expander as recitedin claim 10, wherein the segment controller generates the first signalin response to an isolation command received from the first SCSI bussegment.
 13. The SCSI expander as recited in claim 12, wherein thesegment controller disables the first signal to exit from the isolationmode.
 14. The SCSI expander as recited in claim 13, wherein the segmentcontroller disables the first signal when the second SCSI bus segment isin a bus free state.
 15. The SCSI expander as recited in claim 13,wherein the first SCSI I/O interface circuit includes a first inputbuffer and a first output buffer and wherein the second SCSI I/Ointerface circuit includes a second input buffer and a second inputbuffer.
 16. The SCSI expander as recited in claim 15, wherein the firstand second input buffers are arranged to drive the communication signalsfor input to the SCSI expander from the first and second SCSI bussegments, respectively, and wherein the first buffers are arranged todrive the communication signals for output from the SCCI expander to thefirst and second SCSI bus segments, respectively.
 17. The SCSI expanderas recited in claim 16, wherein the first signal disables the first andsecond output buffers to disable the output of communication signals tothe first and second SCSI bus segments, respectively.